With growing emphasis on reducing energy consumption worldwide, energy conservation and emission reduction have become the focus of attention of various countries. Compared to a non-permanent magnet motor, a permanent magnet motor can improve energy efficiency ratio. In order to reduce the energy consumption, neodymium-iron-boron (Nd—Fe—B) permanent magnet material has been used to produce the motors in the fields of air condition compressors, electric vehicle, hybrid vehicle or the like. Due to a high operating temperature of these motors, a higher intrinsic coercive force is required in all of these magnets. In addition, to increase magnetic flux density of the motors, the magnets were also required to have a higher magnetic energy product.
With conventional manufacture process of a neodymium-iron-boron magnet, it is difficult to satisfy both requirements of a high magnetic energy product and a high intrinsic coercive force. Even if such requirements are achieved, a large amount of rare earth elements Dy and Tb are still needed. However, the reserves of dysprosium (Dy) and terbium (Tb) around the world are finite, thus extensive use of Dy and Tb may cause prices to rise and accelerated depletion of the rare earth resources.
To improve the performance of the permanent magnet material and reduce the use of rare earth, a lot of researches have been done in the art. For instance, CN101404195A disclosed a method for preparing a rare earth permanent magnet by providing a sintered magnet body consisting of 12-17 at % of rare earth, 3-15 at % of B, 0.01-11 at % of metal element, 0.1-4 at % of O, 0.05-3 at % of C, 0.01-1 at % of N, and the balance of Fe, disposing on the surface of the magnet body a powder comprising an oxide, fluoride and/or oxyfluoride of another rare earth, and heat treating the powder-covered magnet body at a temperature below the sintering temperature in vacuum or in an inert gas, so that the other rare earth is absorbed in the magnet body. The characteristic of this method is to accomplish infiltration by disposing an oxide, fluoride and/or oxyfluoride of heavy rare earth, while the disadvantage thereof is to introduce harmful substance such as O and F. into the magnet Above all, after infiltration is completed, substances similar to oxide-scales on the surface of the magnet may arise, which need to be grinded, thus making the magnet materials being wasted.
As another example, CN101506919A disclosed a process for producing a permanent magnet, in which without deteriorating of the surface of the Nd—Fe—B sintered magnet, Dy is efficiently diffused in the crystal grain boundary phase to thereby attain effective enhancements of magnetization intensity and coercive force, and in which post-processes can be avoided. First of all, in a treatment chamber, the Nd—Fe—B sintered magnet and Dy are disposed with an interspace therebetween. Subsequently, in a reduced pressure, the treatment chamber is heated so that not only is the temperature of the sintered magnet raised to a given temperature but also Dy is evaporated to thereby attain supply of evaporated Dy atoms to the surface of the sintered magnet and adhesion therebetween. In this stage, the rate of Dy atoms supplied to the sintered magnet is controlled so that prior to the formation of any Dy layer on the surface of the sintered magnet, Dy is diffused in the crystal grain boundary phase of the sintered magnet. The characteristic of this method is to heat a substance containing heavy rare earth to form a vapor, while the disadvantage thereof is the expensive equipment cost and low evaporation efficiency. The actual comparative result shows that the effect of improving intrinsic coercive force (Hcj) of the latter method is inferior to the former one.
CN101615459A disclosed a method for improving magnetic performance of the sintered neodymium-iron-boron permanent magnet by a strip casting process with grain boundary diffusion of a heavy rare earth compound, wherein the infiltration treatment is prior to sintering. But the disadvantage of the method is that the heavy rare earth which has been enriched between boundary phases may disperse into the interior of the main phase during the high temperature sintering process after the magnet is infiltrated, thereby leading to equalization of heavy rare earth with a poor effect.